Estimating Percent Body Fat
Assessing body composition involves estimating percent body fat. Unfortunately, an autopsy the dissection and chemical analysis of the body is the only method for directly measuring the percentage of body weight that is fat. However, there are indirect techniques that can provide an estimate of percent body fat. One of the most accurate is underwater weighing. Other techniques include skinfold
All of these methods have a margin of error, so it is important not to focus too much on precise values. For example, underwater weighing has a margin of error of about ±3%, meaning that if a person’s percent body fat is actually 17%, the test result could range from 14-20%. The results of different methods may also vary. If you plan to track changes in body composition over time, be sure to perform the assessment using the same method each time. See Table 6.2 for body composition ratings based on percent body fat as a criterion for obesity. It should be noted, however, that the National Institutes of Health has not developed an official standard linking body fat percentage with obesity.
Underwater Weighing In hydrostatic (underwater) weighing, an individual is submerged and weighed under water. The percentages of fat and fat-free weight are calculated from body density. Muscle has a higher density and fat a lower density than water. Therefore, people with more body fat tend to float and weigh less under water, and lean people tend to sink and weigh more under water. Most university exercise physiology departments or sports medicine laboratories have an underwater weighing facility. For an accurate assessment of your body composition, find a place that does underwater weighing or has a BodPod (described in the next section).
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The Bod Pod The Bod Pod, a small chamber containing computerized sensors, measures body composition by air displacement. The technique’s technical name is plethysmography. It determines the percentage of fat by calculating body density from how much air is displaced by the person sitting inside the chamber. The Bod Pod has an error rate of about ±2-4% in determining percent body fat.
Skinfold Measurements Skinfold measurement is a simple, inexpensive, and practical way to assess body composition. Equations can link the thickness of skinfolds at various sites to percent body fat calculations from more precise laboratory techniques.
Skinfold assessment typically involves measuring the thickness of skinfolds at several different places on the body. You can sum the skinfold values as an indirect measure of body fatness. For example, if you plan to create a fitness (and dietary change) program to improve body composition, you can compare the sum of skinfold values over time as an indicator of your program’s progress and of improvements in body composition. You can also plug your skinfold values into equations like those in Lab 6.1 that predict percent body fat. When using these equations, however, remember that they have a fairly substantial margin of error (±4% if performed by a skilled technician), so don’t focus too much on specific values. The sum represents only a relative measure of body fatness.
Skinfolds are measured with a device called a caliper, which is a pair of spring-loaded, calibrated jaws. High-quality calipers are made of metal and have parallel jaw surfaces and constant spring tension. Inexpensive plastic calipers are also available, but you need to make sure they are spring-loaded and have metal jaws to ensure accuracy. Refer to Lab 6.1 for instructions on how to take skinfold measurements. Taking accurate measurements with calipers requires patience, experience, and considerable practice. It’s best to take several measurements at each site (or have several different people take each measurement). Be sure to take the measurements in the exact location called for in the procedure. Because the amount of water in your body changes during the day, skinfold measurements taken in the morning and evening often differ. If you repeat the measurements in the future to track changes in your body composition, measure skinfolds at approximately the same time of day.
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